1. Technical Field
The present invention relates to an LED lightbulb.
2. Description of the Related Art
LED lightbulbs utilizing LED chips as a light source have been proposed as substitutes for incandescent lightbulbs, as disclosed in JP-A-2011-70972, for example. A typical LED lightbulb includes a light emitting unit including a plurality of LED chips, a globe covering the light emitting unit, and a power source unit that converts alternating current into direct current for the LED chips.
An incandescent lightbulb emits light substantially in all directions. Accordingly, the LED lightbulb is required to illuminate a region as broad as possible, when employed as substitute for the incandescent lightbulb. In addition, if the LED lightbulb were remarkably bigger than the incandescent lightbulb, it would be difficult to handle the LED lightbulb like handling the incandescent lightbulb. Therefore, the LED lightbulb is required to be made smaller.
The LED lightbulb including the LED chips mounted therein is starting to take the place of the incandescent lightbulb. The LED lightbulb is superior to the incandescent lightbulb in energy saving performance and life span.
FIG. 47 depicts an existing LED lightbulb (see, for example, JP-A-2010-225409). The LED lightbulb 910 shown in FIG. 47 is designed for use as substitute for an incandescent lightbulb, and includes a substrate 911, a plurality of LED chips 912, a heat dissipator 913, a base 914, and a globe 915. The plurality of LED chips 912 serve as the light source of the LED lightbulb 910, and is mounted on the substrate 911. The substrate 911 is formed of an insulative material, and fixed to the heat dissipator 913. The heat dissipator 913 is formed of a metal such as aluminum. The heat dissipator 913 supports the LED chips 912 via the substrate 911, and serves to release the heat of the LED chips 912 to outside. The base 914 is used to attach the LED lightbulb 910 to an illumination instrument, and formed in compliance with, for example, JIS. The globe 915 covers the plurality of LED chips 912 and transmits the light from the LED chips 912. The globe 915 is formed so as to swell upward in FIG. 47, and fixed to an upper end portion (fixing facet) of the heat dissipator 913.
When the LED lightbulb 910 attached to an illumination instrument is turned on, heat generated by the plurality of LED chips 912 transfers to the heat dissipator 913, thus increasing the temperature of the surface of the heat dissipator 913. Especially, the upper end portion of the heat dissipator 913, close to the LED chips 912 and where the globe 915 is attached is prone to bear a higher temperature. Now, when the LED lightbulb 910 is to be removed from the illumination instrument, the user often grabs the outer circumferential portion of the upper end portion of the heat dissipator 913 opposite the base 914, and/or an adjacent portion, i.e., the outer circumferential portion of the lower end portion of the globe 915. However, as stated above, the heat dissipator 913 (upper end portion in particular) bears a high temperature while the LED lightbulb 910 is turned on, and hence the surface of the upper end portion of the heat dissipator 913 maintains the high temperature for a while after the LED lightbulb 910 is turned off. In the case where the user touches the outer circumferential portion of the upper end portion of the heat dissipator 913 while it is still hot in order to remove the LED lightbulb 910, the user may feel uncomfortable.
FIG. 48 depicts another example of existing LED lightbulbs (see, for example, JP-A-2009-267082). The LED lightbulb 920 shown in FIG. 48 includes a plurality of LED modules 922 mounted on a substrate 921. The LED modules 922 have the function of emitting light from the LED lightbulb 920, and each include an LED chip (not shown). The substrate 921 is supported by a heat dissipator 923. A base 925 is used to attach the LED lightbulb 920 to an illumination instrument designed for an incandescent lightbulb. A globe 926 is formed in a generally spherical shape, and transmits light from the LED modules 922. A power source unit 924 supplies power to the LED modules 922. The power from the power source unit 924 is supplied to the LED module 922 through wiring 927. An end portion of the wiring 927 is connected to the upper surface of the substrate 921 by means of a solder 928.
The solder 928 is typically formed with a flux. The flux often contains bromine. While the LED lightbulb 920 is in use, the bromine is emitted from the flux. The bromine may provoke a change of color of a metal constituting the LED modules 922, which may lead to decreased light amount of the LED lightbulb 920.
As stated above, the LED lightbulbs including the LED chips mounted therein have been proposed as substitute for incandescent lightbulbs, because of the advantage in energy saving performance and life span compared with the incandescent lightbulbs.
The LED lightbulb has an appearance similar to that of the conventional incandescent lightbulb, and the base of the LED lightbulb serving as the power supply terminal is also equivalent to that of the incandescent lightbulb. Therefore, the LED lightbulb can be fitted to existing sockets designed for the incandescent lightbulb.
The LED lightbulb includes LED modules and a lighting circuit as disclosed, for example, in JP-A-2011-70972 and JP-A-2011-82132, so as to be turned on upon receipt of power from the lighting circuit.
However, it has been difficult to control the light of the LED lightbulb through a simple procedure. The existing LED lightbulbs are not provided with the dimming control function. In addition, the LED is driven by a constant current as disclosed in JP-A-2011-70972, and therefore the dimming control is performed by changing the duty ratio of the current pulse signal. For such a reason, even though the LED lightbulb can be fitted to a socket for the conventional incandescent lightbulb, the dimming control system for the incandescent lightbulb is inapplicable to the LED lightbulb, because the dimming control method is different.